[MSHARED-1079] add build() method and document toString() method

improve Javadoc

src/main/java/org/apache/maven/shared/utils/logging/AnsiMessageBuilder.java

@@ -150,6 +150,12 @@ public AnsiMessageBuilder format( String pattern, Object... args )
 
     @Override
     public String toString()
+    {
+        return build();
+    }
+
+    @Override
+    public String build()
     {
         return ansi.toString();
     }

src/main/java/org/apache/maven/shared/utils/logging/MessageBuilder.java

@@ -21,6 +21,8 @@
 
 /**
  * Message builder that supports configurable styling.
+ * An instance of this interface can be retrieved with {@link MessageUtils#buffer()}.
+ * After the message has been build with any of the append methods its content can be retrieved with {@link #build()}.
  * @see MessageUtils
  * @since 3.1.0
  */
@@ -130,4 +132,20 @@ public interface MessageBuilder
      * @return the current builder
      */
     MessageBuilder format( String pattern, Object... args );
+
+    /**
+     * Get the message constructed by this builder.
+     * The underlying buffer is not reset with this method, i.e. if you continue using this builder you just
+     * append content to the existing one.
+     * @return the message
+     * @since 4.0.0
+     */
+    String build();
+
+    /**
+     * Same as {@link MessageBuilder#build()}.
+     * @deprecated Rather use {@link MessageBuilder#build()}
+     */
+    @Deprecated
+    String toString();
 }

src/main/java/org/apache/maven/shared/utils/logging/PlainMessageBuilder.java

@@ -136,6 +136,12 @@ public PlainMessageBuilder format( String pattern, Object... args )
 
     @Override
     public String toString()
+    {
+        return build();
+    }
+
+    @Override
+    public String build()
     {
         return buffer.toString();
     }